Absorbent articles for the absorption of fluids aim to rapidly absorb fluids so that they are not left in contact with the user. Conventional Porous Media (PM) structures and super absorbent polymers (SAP) utilized as acquisition and storage layers in the hygiene product industry show a known trade-off between driving force for fluid acquisition (capillary suction) and resistance to the flow (inverse of permeability). This is due to the underlying physics behind flow into the desired material, which can be effectively described by statistical percolation theories.
Ultimately, the material structure is responsible for both driving force and resistance to flow in such a way that whenever the structure presents high surface/volume ratios the capillary suction increases but the permeability decreases, because the flow becomes more tortuous. Conversely, whenever the ratio surface to volume is low in a porous material, then the resistance to flow is reduced (high permeability) at the expenses of the capillary suction.
This dichotomy is effectively represented by a single capillary tube model. By increasing the radius of the capillary, one can significantly speed up the capillary rise against gravity (representing our acquisition process) at the expense of the driving force (capillary pressure). This is shown as reduced equilibrium pressure (which in the case of absorbent products translates to lower rewet pressure and hence less secure storage). On the other hand, decreasing the capillary radius allows to securely store more fluid against pressure at the expense of the speed of acquisition as the capillary rise process becomes slower.
As such there exists a need to create an absorbent structure that breaks the tradeoff between capillarity and permeability in a single stratum to create a product that exhibits both high capillarity and high permeability.